1. Field of the Invention
The present invention relates to a panoramic X-ray imaging apparatus for performing tomographic imaging of the entire jaws of a human head for the purpose of dental diagnosis and treatment.
2. Description of the Related Arts
As related art of an X-ray imaging apparatus for performing tomographic imaging of the entire jaws of a human head for the purpose of dental diagnosis and treatment, prior art wherein the feeding speed of an X-ray film is adjusted is, for example, disclosed in Japanese Laid-open Patent Application No. Sho 57-31844. Japanese Laid-open Patent Application No. Sho 60-163642 discloses that the feeding speed of an X-ray film is adjusted in accordance with the curvature of a dental arch. Japanese Laid-open Patent Application No. Sho 57-166144 discloses prior art wherein X-rays are detected by using an X-ray image sensor, such as a CCD, and electrical image processing is performed, and then an image is displayed. Japanese Laid-open Patent Application No. Sho 61-22841 discloses prior art wherein an X-ray image formed on a CCD sensor while being moved is subjected to time delay integration in accordance with the movement by technology referred to as "Time Delay Integration (TDI)."
Furthermore, Japanese Utility Model Publication No. Hei 4-48169 and Japanese Laid-open Patent Application No. Hei 2-84942 disclose prior art wherein panoramic tomographic imaging of the jaws of a patient is performed by driving a CCD sensor by using the TDI method. In these references, an X-ray CCD sensor is used instead of an imaging film adapted to a conventional panoramic X-ray apparatus. In other words, at both ends of a horizontal arm which is rotatable around its vertical axis, an X-ray source and a CCD sensor are disposed opposite to each other with the head of a patient positioned between them. The X-ray beam generated from the X-ray source is narrowed by a primary slit and passes through the head of the patient, and is then narrowed by a secondary slit. The X-ray image at the secondary slit is converted into a visible light image and enters the CCD sensor. Signal charges are stored while the delay time of a vertical transfer clock signal for shifting at vertical shift registers is adjusted in accordance with the movement of the image at the secondary slit. As a result, a desired tomographic image by the TDI method can be obtained.
In Japanese Utility Model Publication No. Hei 4-48169 and Japanese Laid-open Patent Application No. Hei 2-84942, the adjustment of the vertical transfer clock signal of the CCD sensor by the TDI method is controlled by an internal CPU. However, the relationship between the clock signal and arm rotation is not disclosed. Accordingly, in these references, it is assumed that the arm rotates at a constant rotation speed and that film feeding speed is set in accordance with the rotation speed.
In case arm rotation is completely independent of the vertical transfer clock signal of the CCD sensor, when the rotation speed of the arm is changed by an unexpected problem while a panoramic X-ray image is taken, the relationship between arm rotation and the vertical transfer clock signal is deviated from a desired relationship, and no clear tomographic image can be obtained. In addition, a slow start function is desired to be added at the start of arm rotation in consideration of the safety of patients and the stability of the apparatus itself. Even when the slow start function is added, unless arm rotation speed is synchronous with the vertical transfer clock signal of the CCD sensor, no clear tomographic image can be obtained. Furthermore, the arm rotation speed may have variations inherent in the apparatus and may change with time. Moreover, the intensity of the X-rays from the X-ray source has upper and lower limits, and the intensity of the X-rays allowed to be applied to a human body is limited. For these reasons, when panoramic X-ray imaging is performed for a patient with a large head, it is necessary to perform imaging at low speed by taking longer time even when the intensity of the X-rays of the X-ray source is at its upper limit. When panoramic X-ray imaging is performed for a patient with a small head, it is necessary to lower the intensity of X-rays and perform imaging by taking shorter time at low X-ray intensity.